X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, in the vicinity of the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. Exemplary systems and methods for XRR are described in U.S. Pat. Nos. 5,619,548, 5,923,720, 6,512,814, 6,639,968, and 6,771,735, whose disclosures are incorporated herein by reference.
Small-angle X-ray scattering (SAXS) is another method for surface layer characterization. It is described, for example, by Parrill et al., in “GISAXS—Glancing Incidence Small Angle X-ray Scattering,” Journal de Physique IV 3 (December, 1993), pages 411-417, which is incorporated herein by reference. In this method, an incident X-ray beam is totally externally reflected from a surface. The evanescent wave within the surface region is scattered by microscopic structures within the region. Measurement of the scattered evanescent wave can provide information about these structures. For example, SAXS can be used in this manner to determine characteristics of pores in a surface layer of a low-k dielectric material formed on a silicon wafer.
U.S. Pat. No. 6,895,075, whose disclosure is incorporated herein by reference, describes methods and systems for performing combined XRR and SAXS measurements on a sample. Although XRR and SAXS are complementary in terms of the information they provide, there are difficulties inherent in performing both types of measurements using a single system. In terms of irradiation of the sample, for precise measurement of SAXS, a collimated beam is advantageous. On the other hand, XRR may advantageously use a converging beam with a large convergence angle, so that reflectivity measurements may be made over a range of several degrees simultaneously. In the embodiments disclosed in U.S. Pat. No. 6,895,075, X-ray inspection apparatus comprises a radiation source, which is configured to irradiate a small area on a surface of a sample. The X-ray optics control the radiation beam so as to adjust the angular width and height of the beam appropriately for XRR or SAXS.
On the detection side, SAXS typically looks at scattering as a function of azimuth, within the surface plane of the sample, while XRR is based on measuring reflected X-rays as a function of elevation, perpendicular to the surface plane. In the embodiments described in U.S. Pat. No. 6,895,075, the detection assembly comprises an array of detector elements, which is positioned to receive radiation that is reflected or scattered from the irradiated area. The array has two operative configurations: one in which the elements of the array resolve the radiation along an axis perpendicular to the plane of the sample, and another in which the elements resolve the radiation along an axis parallel to the plane. The appropriate configuration is selected, mechanically or electronically, for the type of measurement being performed.
X-ray diffractometry (XRD) is a well-known technique for studying the crystalline structure of matter. In XRD, a sample is irradiated by a monochromatic X-ray beam, and the locations and intensities of the diffraction peaks are measured. The characteristic scattering angles and the scattered intensity depend on the lattice planes of the sample under study and the atoms that occupy those planes. For a given wavelength λ and lattice plane spacing d, diffraction peaks will be observed when the X-ray beam is incident on a lattice plane at angles θ that satisfy the Bragg condition: nλ=2d sin θ, wherein n is the scattering order. The angle θ that satisfies the Bragg condition is known as the Bragg angle. Distortions in the lattice planes due to stress, solid solution, or other effects lead to observable changes in the XRD spectrum.
XRD has been used, inter alia, for measuring characteristics of crystalline layers produced on semiconductor wafers. For example, Bowen et al. describe a method for measuring germanium concentration in a SiGe structure using high-resolution XRD in “X-Ray metrology by Diffraction and Reflectivity,” Characterization and Metrology for ULSI Technology, 2000 International Conference (American Institute of Physics, 2001), which is incorporated herein by reference.
XRD may also be used at grazing incidence to observe structures on the surface of a sample. For example, Goorsky et al. describe the use of grazing-incidence XRD for analyzing epitaxial layer structures on a semiconductor wafer in “Grazing Incidence In-plane Diffraction Measurement of In-plane Mosaic with Microfocus X-ray Tubes,” Crystal Research and Technology 37:7 (2002), pages 645-653, which is incorporated herein by reference. The authors apply the technique to determine the in-plane lattice parameter and lattice orientation of very thin surface and buried semiconductor layers.
In the context of the present patent application and in the claims, the terms “scatter” and “scattering” are used to refer to any and all processes by which X-ray irradiation of a sample causes X-rays to be emitted from the sample. Thus, in this context, “scattering” includes the phenomena of XRR, XRD and SAXS, as well as other scattering phenomena known in the art, such as X-ray fluorescence (XRF). On the other hand, the specific term “small-angle X-ray scattering,” abbreviated SAXS, refers to the particular phenomenon of grazing-incidence scattering in the sample plane, as described above.